In the same year the Duchy secretly bought £86,000 shares in Bermuda-based Sustaina­ble Fo­restry Management Ltd, where Charles’ late pal Hugh van Cutsem was a director.

Weeks later the prince, 68, began campaigning on environmental agreements which would boost the firm’s shares, without declaring his investment, papers say.

Charles’ estate almost tripled its money in just over a year, although it is not clear why.

The Duchy’s actions in both investments are legal and there is no suggestion of tax avoidance.

The Duchy of Cornwall invested £86,000 in a Bermuda-based firm

The Duchy’s annual report said Charles is “actively involved” in its running.

But a spokesman said today he “does not have any direct involvement in investment decisions”.

A Clarence House spokesman insisted the prince “has never chosen to speak out on a topic simply because of a company that the Duchy may have invested in”.

But Sir Alistair Graham, ex-chairman of the Committee on Standards in Public Life, said: “There’s a conflict of interest between his own investments of the Duchy of Cornwall and what he’s trying to achieve publicly.

“It’s unfortunate somebody of his importance, his influence, becomes involved in such a serious conflict.”

Shadow Chancellor John McDonnell calls for public inquiry after Paradise Papers leak which included funds managed for The Queen

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